Abstract:

A sandwich structure reinforcement carried out by a gripper, which pierces
the sandwich structure or the core material, only at one side, thereby
obtaining a hole penetrating through, e.g., a polymer rigid foam. At the
opposite side, the gripper catches the reinforcing structure (for example
a sewing thread, pultruded fiber-plastic composite rods) and inserts the
reinforcing structure into the sandwich structure during back movement
thereof. The through hole is additionally enlargeable by the reinforcing
structure, thereby making it possible to obtain an important fiber volume
part in the through hole of the core material.

Claims:

1-9. (canceled)

10: A reinforcing process for a core composite, comprising:a gripper
system making an insertion from one side of a structure into a core
material or into the core material with cover layers applied, on an
opposite side gripping a reinforcing structure and, by a backward
movement, introducing the reinforcing structure into the core material or
into the core material with cover layers applied.

11: A reinforcing process for a core composite according to claim 10,
wherein the reinforcing structure includes textile-like strengthening
structures or elements in bar form.

12: A reinforcing process for a core composite according to claim 10,
wherein the cover layers include textile semifinished products, the core
material of polymeric, natural or textured core material, and wherein the
cover layers, the core material and the reinforcement elements are
embedded in a polymeric matrix material.

13: A reinforcing process for a core composite according to claim 10,
wherein the reinforcing structure is not cut to length after introduction
into the core material or into the core material with cover layers
applied.

14. A reinforcing process for a core composite, according to claim 10,
wherein the reinforcing structure is cut to length after introduction
into the core material or into the core material with cover layers
applied.

15: A core composite, obtainable by a method of claim 10.

16: Use of the core composite according to claim 15 for production of
spacecraft, aircraft, sea and land craft, and rail vehicles.

17: Use of the core composite according to claim 15 for production of
sports equipment.

18: Use of the core composite according to claim 15 for production of
structural elements for interior, trade-fair, and exterior construction.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001]The invention relates to the design and production of reinforcement
elements that traverse the thickness of the core composite according to
the preamble of claim 1 for strengthening core composite structures.

[0003]With the aid of this invention, the transversal properties (for
example compressive or tensile rigidity and strength in the z direction,
shear rigidity and strength in the xz and yz planes, peel resistance
between cover layer and core, failsafe behaviour) and also the in-plane
mechanical properties of core composite structures (for example rigidity
and strength in the direction of the plane of the sheet) can be increased
significantly with the aid of reinforcement elements that traverse the
thickness.

PRIOR ART

[0004]All previously known production methods for reinforcing core
composite structures in the direction of their thickness, such as for
example the double-saddle-stitch, blind-stitch or two-needle stitching
technique and the tufting method, have the common feature that the
reinforcement elements (for example stitching threads, rovings) are
introduced into the core composite structure together with the needle. In
the case of conventional textile-like stitched materials, the penetration
of the needle including the stitching thread and the subsequent pulling
out of the stitching needle and leaving behind of the stitching thread in
the stitching hole generally do not present any problem on account of the
resilient effect of the textiles. However, in the case of core composite
structures with a polymeric rigid foam as the core material, the
penetration of the needle including the stitching thread causes the
cellular structure to be destroyed and the polymeric rigid foam to be
deformed to the size of the stitching needle diameter as a result of
plastic and elastic deformation.

[0005]Once the stitching needle has been pulled out and the stitching
thread left behind in the stitching hole, there is a reduction in the
through-hole on account of the elastic deformation components of the cell
walls, whereby the core hole diameter again becomes smaller again than
the stitching needle diameter (see FIG. 2). There is a virtually linear
dependence between the diameter of the through-hole in the core that is
obtained and the stitching needle diameter that is used (FIG. 2), i.e.
the greater the stitching needle diameter, the greater too the resultant
through-hole in the core. Furthermore, the stitching thread causes
additional widening of the core hole diameter. This additional widening
corresponds approximately to the cross-sectional area of the stitching
thread (FIG. 2). It is also the case here that, the greater the
cross-sectional area of the stitching thread used, the greater the
additional widening.

[0006]After impregnation of the core composite structure with the liquid
matrix material and subsequent curing, the core hole diameter and the
fibre volume content of the stitching thread in the core hole can be
determined by means of microscopic examinations. Experimental
examinations on core composite structures stitched by means of
double-saddle-stitch stitching technology and when using a stitching
needle with a diameter of 1.2 mm and an aramid thread with a line weight
of 62 g/km show here that the diameter of the resin column obtained in
the core material (about 1.7 mm) is greater than the determined core hole
diameter of a non-impregnated core composite structure (about 1.1 mm;
compare FIGS. 2 and 3) in the case of single insertion. The reason for
this is that adjacent cell walls in the region of the stitching needle
diameter are destroyed by the insertion of the stitching needle. In the
subsequent infiltration process, resin can then penetrate into these then
open pores with an average diameter of about 0.7 mm (FIG. 4).

[0007]When the double-saddle-stitch stitching technique is used, with each
insertion two stitching threads are always introduced in the z direction
of the core composite structure (see FIGS. 4 and 5). In order to increase
the stitching thread volume content within a through-hole, and
consequently the reinforcing effect, already stitched places can be
stitched once more or a number of times. However, stitching threads that
are already in the core hole may be damaged by the renewed insertion of
the stitching needle. With the aid of microscopic examinations, it can be
established that the stitching thread volume content may not be increased
in proportion to the number of insertions, as would be expected (FIGS. 3,
4 and 5). The reason for this is that the diameter of the core hole does
not remain constant as the number of insertions and the stitching threads
introduced increase, since the core hole diameter is increased by the
additional introduction of stitching threads by approximately the
cross-sectional area of the threads (FIG. 3, dashed curve). However, it
is likewise established that the true curve profile (FIG. 3, solid curve)
only obeys this theory when there is a very high number of insertions. By
contrast, when there is a small number of insertions, the diameter of the
core hole increases to a disproportionately great extent. The reason for
this is the positioning accuracy of the stitching machine. If a position
which is to be stitched once again is moved to again, the stitching
needle is not inserted precisely centrally into the already existing hole
but a little to the side, within the limits of positioning accuracy,
whereby the core hole is increased disproportionately. After insertion
into the same core hole approximately eight times, the said hole has
already been widened to such an extent that the stitching needle enters
the existing hole without additional destruction of cell walls. With
further insertions, the widening only takes place as a result of the
additional stitching threads that are introduced. In FIGS. 4 and 5 there
is shown the possible increase in the stitching thread volume content as
the number of stitching threads in the core hole increases. The black
curve in FIG. 4 describes the proportional increase of the stitching
thread volume content with a constant core hole diameter, the dash-dotted
curve describes it on the basis of the aforementioned theory of exact
positioning accuracy and the additional widening of the core hole
diameter as a result of the stitching threads introduced and the dotted
curve describes the true profile of the stitching thread volume content
as the number of stitching threads or insertions increases. In the case
of single insertion, only a fibre volume content of about 3.2% can be
achieved, which can be increased only to about 20% by insertion up to 10
times (see FIGS. 4 and 5). By contrast, the fibre volume content of a
single stitching thread strand is about 58% (see FIG. 4).

[0008]It is clear from these examinations that the diameter obtained in
the polymer core material when using conventional production methods (for
example double-saddle-stitch stitching technology) is mainly dependent on
the stitching needle diameter used, the cross-sectional area of the
stitching thread and the core diameter of the polymeric rigid foam used.
Since in the case of all the previously known reinforcing methods
stitching needles and stitching threads are inserted simultaneously into
the core composite structure, there is always an unfavourable
relationship between the cross-sectional area of reinforcement elements
that is introduced and the size of the core hole diameter. High fibre
volume content in the core hole diameter, similarly high to the fibre
volume content of the cover layers (greater than 50%), consequently
cannot be achieved with conventional reinforcing methods. Since, however,
the mechanical properties are mainly influenced by the high-rigidity and
high-strength reinforcement elements that are introduced, the aim must be
to strive for a fibre volume content of the reinforcement in the core
hole diameter that is as high as possible. Furthermore, the high resin
component in the core hole diameter causes an increase in the weight,
which in the aerospace sector in particular is not tolerated.

OBJECT

[0009]The invention is based on the object of improving the mechanical
properties of core composite structures by incorporating reinforcement
elements in the direction of the thickness of the core composite
structure (z direction), with the possibility of achieving a high fibre
volume content of the reinforcement in the core hole diameter.
Furthermore, the weight is not to be adversely influenced too much by the
incorporation of the reinforcement elements in the core composite
structure. This novel stitching technique may likewise be used for
preforming and fastening additional structural components (for example
stringers, frames etc.) to the core composite structure.

SOLUTION

[0010]This object is achieved by the introduction of a necessary
through-hole in the core material and the introduction of the reinforcing
structure taking place at different times from each other, whereby the
fibre volume content of the reinforcement in the core hole diameter can
be adjusted by the cross-sectional area of the stitching thread that is
used. FIG. 1 illustrates the basic invention and design of a core
composite structure reinforced in such a way. A gripper system (2) makes
a unilateral insertion from one side of the core composite structure
(steps 1 and 2) into the core material (4) and optionally through the
upper textile cover layer (3) and lower textile cover layer (5) (step 2)
and, with the aid of a gripper (1), receives on the opposite side a
reinforcing structure (6), for example stitching thread, pultruded
fibre-plastic-reinforced bars, which are supplied by means of a device
(7), (step 2), and introduces the reinforcing structure into the core
composite structure during the backward movement (step 3). In the
subsequent process step, the gripper system (2) moves upwards and draws
the reinforcing structure into the core or into the core composite
structure (step 3).

[0011]A polymeric rigid foam (for example PMI, PVC, PEI, PU etc.) may be
used as the core material (4). The core material (4) may have a thickness
of up to 150 mm, a width of about 1250 mm and a length of 2500 mm. The
upper textile cover layer (3) and the lower textile cover layer (5) may
be constructed identically or differently and consist of glass, carbon,
aramid or other strengthening materials. The thickness of an individual
textile cover layer ply may be identical or different and lie between 0.1
mm and 1.0 mm. Thermoplastic or thermosetting materials may be used as
the polymeric matrix material.

[0013]In the subsequent process step, the stitched material or the
reinforcing unit is transported further to the next insertion position
and the reinforcing process is then repeated there. In addition, the
supplied reinforcing structure may be cut to length, so that there is no
link from one insertion to the other. The cutting to length may be
performed by all customary technical means, such as for example by
mechanical cutting or flame cutting. The drawing-in of the reinforcing
structure can cause additional widening of the core hole diameter
obtained by the insertion of the gripper system, whereby a high fibre
volume content can be realized. Since the reinforcement elements are
introduced into the core composite structure or only into the core
material by tension, there is very good alignment and no buckling of the
strengthening structure. With the aid of this reinforcing method, the
incorporated reinforcement elements may likewise have an angle other than
0° in relation to the z axis, for example +/-45°, under
loading with purely transverse force.

[0014]The use of core composite structures that are strengthed in the
direction of their thickness according to the invention can be used in
the transport sector, such as for example in aerospace, motor vehicle and
rail vehicle construction and in shipbuilding, but also in the sport and
medical sectors as well as in the building trade.

[0015]After the reinforcing process, the core composite structure may be
impregnated with a thermosetting or thermoplastic matrix material in a
liquid-composite-moulding process.